Which action is recommended regarding altitude change to avoid jet stream turbulence?

The recommended action to descend if the ambient temperature is falling is based on the understanding of jet streams and the associated turbulence characteristics. When ambient temperatures fall, it typically indicates that the aircraft is flying into a region of colder air, which can be associated with stronger winds and turbulence typically found in the vicinity of jet streams.

Descending into warmer air or altitudes where temperatures are rising can help avoid the shear and turbulence that are often more prevalent at higher altitudes where jet streams are located. The cooler temperatures can indicate a higher probability of encountering turbulence from the jet stream, especially at higher altitudes where these winds are more intense. Therefore, by choosing to descend when temperatures are falling, pilots can potentially experience a smoother ride by moving away from these turbulent zones associated with colder air and jet streams.

The other options may present varying scenarios regarding altitude changes but do not directly address the turbulence caused by jet streams in the context of temperature changes effectively. Specifically, maintaining altitude or climbing does not offer the same proactive strategy to mitigate the likelihood of encountering jet stream turbulence.